US3758809A

US3758809A - Emissive fused pellet electrode

Info

Publication number: US3758809A
Application number: US00204478A
Authority: US
Inventors: R Menelly; E Kern
Original assignee: Deutsche ITT Industries GmbH
Current assignee: TDK Micronas GmbH; ITT Inc
Priority date: 1971-06-07
Filing date: 1971-12-03
Publication date: 1973-09-11
Anticipated expiration: 1990-09-11

Abstract

An emissive electrode of the type utilized in fluorescent lamps and a method for making such an electrode. The electrode includes a fused pellet and a metal lead extending from the bottom surface thereof. The pellet contains a fused mixture of electron emissive material and a metal having a high melting point and a low vapor pressure. The pellet has a bulk density gradient structure wherein the interior portions and exterior bottom and side portions thereof have a high bulk density relative to the bulk density of the exterior top portion of the pellet, and the exterior bottom and side surfaces of the pellet are smooth re1ative to the exterior top surface thereof. The pellet is formed by mixing a powder of said metal with a powder of said electron emissive material, compressing said mixed powder and heating it in a mold until an exothermic reaction occurs.

Description

United States Patent [1 1 Menelly et a1.

EMISSIVE FUSED PELLET ELECTRODE Inventors: Richard A. Menelly, Danvers, Mass.;

Edmund R. Kern, Hampton, N.H.

Assignee: international Telephone and Telegraph Corporation, Nutley, NJ.

Filed: Dec. 3, 1971 Appl. No.: 204,478

Related U.S. Application Data [n1 3,758,8fi9

1 51 Sept. 11, 1973 Primary Examiner-David Schonberg Assistant Examiner-Paul A. Sacher Att0rneyC. Cornell Remsen, Jr. et al.

[57] ABSTRACT An emissive electrode of the type utilized in fluorescent lamps and a method for making such an electrode. The electrode includes a fused pellet and a metal lead extending from the bottom surface thereof. The pellet contains a fused mixture of electron emissive material and a metal having a high melting point and a low vapor pressure. The pellet has a bulk density gradient structure wherein the interior portions and exterior bottom and side portions thereof have a high bulk density relative to the bulk density of the exterior top portion of the pellet, and the exterior bottom and side surfaces of the pellet are smooth relative to the exterior top surface thereof. The pellet is formed by mixing a powder of said metal with a powder'of said electron emissive material, compressing said mixed powder and heating it in a mold until an exothermic reaction occurs.

10 Claims, 3 Drawing Figures 1. EMISSIVE FUSED PELLET ELECTRODE CROSS-REFERENCE TO RELATED APPLICATIONS This is a conti'nuation-in-part of our copending application, Ser. No. 150,311 U.S.-Pat. No. 3,502,855 entitled Improved Fused Pellet Electrode," filed June 7, 1971 and it is related to copending application Ser. No. 204,469, entitled Em-issive Electrode, filed on even date herewith and-assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION This invention relates to emissive fused pellet electrodes of the cold cathode type and to a method of making such electrodes, and more particularly to such electrodes having a bulk density gradient structure.

This invention is an improvement over the cold cathode described and claimed in the abovementioned related patent application, Ser. No. 204,469, and reference may be made thereto for a general discussion of the structure and operation of the two types of cathodes presently utilized in the fluorescent lamp art, summarizing briefly, both of said two types of cathodes are, for operation, heated to what is termed the thermionic emission temperature," at which temperature they emit electrons. However, for the purposes of this application, as was the case with regard to said related application, one of said cathode types is termed a hot cathode, this cathode being of the type which. is heated to its emission temperature by a heated filament, whereas the otherof said two types of cathodes is termed a cold cathode, this cathode being of the type which is heated to its emission temperature by ionic bombardment. Further of interest is a specie of the cold cathode which is termed, for the .purposes of this specification, a hybrid cathode, this cathode being one which has a structure similar to that of the hot cathode but which is heated'to its thermionic emission temperature inthe same manner as the cold cathode.

To overcome the unsatisfactory features of the cold cathodes known in the art, these unsatisfactory features having been fully discussed in said related application, Ser. No. 204,469, that is, their excessive ignition voltage requirements and their extended glow to arc transition periods, during which undesirable sputtering occurs, a fused pellet electrode, as disclosed in said related patent application, was invented. This fused pellet electrode, while providing performance substantially superior to that of the cold cathodes known in the art .was itself found to be unsatisfactory in a number of re-- spects. Firstly, although it caused less end-blackening in the fluorescent lamps in which it was utilized than the cold cathodes previously known, it still caused more of said end-blackening than is desirable. Secondly, although the ignition voltage of this fused pellet electrode is approximately equal to that required by hybrid cathodes, it is desirable to reduce the ignition voltage required still further..Thirdly, the fused pellet electrode was found to be not as structurally sound as the previously mentioned cold cathodes due to the fact that the fused pellet electrode did not utilize a container for retaining the emissive material as did said cold cathodes, and therefore the fused pellet cathode was not preferred for use where there was likelihood of lamp and electrode vibration. Finally, since as previously stated the fused pellet electrode did not utilize a container for retaining its emissive material, the required conducting electrode lead was connected directly to the fused pellet by embedding said lead within the pellet and this resulted in an electrical connection which was found to be inferior to the connection obtainable with the aforementioned cold cathodes, the conducting electrode lead there being firmly connected to the container retaining the emissive material.

SUMMARY OF THE INVENTION Therefore, the main objectof this invention is to provide an emissive fused pellet cathode having a bulk density gradient structure which has a lower ignition voltage than presently known fused pellet cathodes.

It is a further object of this invention to provide such a cathode which causes less end-blackening in the fluorescent lamp in which it is utilized than presently known fused pellet cathodes.

It is yet another object of this invention to provide such a cathode which is mechanically sounder than presently known fused pellet electrodes.

It is yet another object of this invention to provide such a cathode which has a superior electrical connection between the conducting electrode lead and the electron emissive material than presently known fused pellet electrodes.

It is still a further object of this invention to provide a method of making such a cathode.

According to the present invention there is provided a fused pellet electrode comprising a fused pellet having a top, a bottom and sides and containing a fused mixture of electron emissive material and a metal, and a metal conducting lead embedded in and extending from the bottom of said pellet, said pellet having a bulk density gradient structure, wherein the interior portions and exterior bottom and side portions of said pellet have a high bulk density relative to the bulk density of the exterior top portion of said pellet.

According to another aspect of the invention, there is provided a method for making a fused pellet electrode comprising the steps of placing a powder mixture comprising a metal powder and a powder of an electron emissive material in a mold, compressing said powder mixture with a pressure in the range of LOGO-4,000 pounds per square inch and heating said powder mixture to a temperature above the melting point of said emissive material and until an exothermic reaction begins, said reaction continuing until self extinguishing, whereupon the fused pellet having a bulk density gradient structure is formed.

It is a feature of this invention that the fused pellet electrode constructed according to the instant method is relatively air stable, that is, it may be exposed to reasonably dry air, by which is meant air containing less than grains of water per pound of dry air, for at least one hour after activation and it is thus suitable for batch activation processes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a mold utilized in constructing the inventiveelectrode;

FIG. 2 illustrates a step in the electrode construction process; and

FIG. 3 is a cross-sectional view of the inventive electrode.

DESCRIPTION oF THE PREFERRED EMBODIMENT to as a transitional metal, and a source of electron emissive material is prepared. In the specific example to be provided below, a-few grams of powdered tantalum, which is arefractory metal, approximately 99.8 percent pure, is mixed 67 percent by weight, with reagent grade barium peroxide powder, 33 percent by weight. Of course, other refractory metal powders, such as tungsten, molybdenum, thorium, titanium, zirconium and mixtures thereof could be used instead of tantalum powder, while other electron emissive materials, for example, alkaline earth metal compositions such as.oxides, peroxides, and nitrates of barium, and oxides, peroxides and nitrates of barium in combination with calcium oxide, strontium oxide and zirconium dioxide, or alkali metal compositions, such as compounds of lithium, cesium, sodium and potassium, could be used instead of barium peroxide. Mixtures of calcium, strontium and zirconium compounds are generally added to barium compounds in order to retard the emission of electrons from, the finally formed fused electrode. However, it should also be noted that the increase in amounts of calcium, strontium and zirconium compounds will tend to increase the work function of the finally fused electrode.

It is appropriate to note at this point that in forming the subject fused pellet electrode it is necessary to control the rate of the exothermic reaction which is used to form the electrode said reaction to be more fully discussed below, since if the rate of the reaction is too great much of the material utilized in forming the electrode will splatter off, resulting in its being lost. If, on the other hand, the rate of reaction is too slow, the density gradient of the resultant electrode will be too small, and the electrode structure will approach homogeneity. Such a homogeneous structure will result in a less efficient cathode than is otherwise obtainable. Further, if the rate 'of the exothermic reaction is too slow the resultant electrode will have a high ignition voltage requirement than is otherwise obtainable since it will require more time to go through the glow to arc transition due to the fact that it will have a more uniform surface,

that is, it will have less protrusions available for starting the required are.

At this time it is well to note that although, as stated above, different refractory metal powders, such as thorium or titanium may be utilized to form the instant fused electrode, these refractory metal powders have a greater exothermicity than, for example, tantalum. Thus, while an electron emissive material such as barium peroxide may here too be utilized in forming the subject cold cathode, it has been found advantageous to use a greater quantity by weight of, for example, zirconium powder'and a lesser quantity of barium peroxide or other alkali or alkaline earth material than would be used if the refractory metal powder was, for example tantalum. A satisfactory reaction rate may'also be obtained if it is desired to use the same quantity by weight of zirconium powder as of tantalum powder, if the less reactive oxide of barium or other alkaline earth metal is substituted for the more reactive peroxide of the alkaline earth metal. In the same manner it has been found that transitional metals, such as nickel and iron, which have a lower exothermicity than tantalum may also be utilized to form the subject cold cathode. This may be accomplished by either utilizing a greater portion of alkali or alkaline earth compound such as barium peroxide than would be used if a refractory metal were used rather than the transitional metal, or alternatively, the same result may be accomplished by keeping the ratio of the transitional metal constant and using a more reactive compound of alkali or alkaline earth metal than barium peroxide, such as for example,barium nitrate. In general, although the above example specified that 67 percent by weight of a refractory metal powder and.33 percent by weight of an alkaline earth compound be used, it has been found through experimentation that ranges of mixtures of approximately 50 percent to percent of metal powder by weight and approximately 20 percent to 50 percent alkaline earth compound by weight may be utilized to produce satisfactory reation rates and therefore may be used to produce satisfactory fused electrodes.

Returning now to the example, the above described mixture is prepared by rolling two parts by weight of tantalum powder and one part by weight of barium peroxide with flint pebbles in a standard porcelain jar mill, for a period of, for example, one hour.

Referring now to FIG. 1, there is illustrated a split mold 1 having

sections

2 and 3. The mold has a cavity 4 formed therein, said cavity being approximately twotenths of an inch in diameter and approximately onefourth inch deep. A hole 5 extends from the bottom central portion of cavity 4, said hole 5 being approxima'tely twenty-five thousandths of an inch in diameter and one-fourth inch deep. Though mold 1 is here illustrated as a split mold, an integral mold maybe utilized as a substitute therefor and the mold is formed in a split configuration only to render the subsequently formed fushed pellet electrode, which is to be discussed below,

more easily removable therefrom. Mold .1 is con-- structed of a conducting material to which the above mentioned fused pellet electrode will not adhere and it is constructed, in this example, of graphite' Referring now to FIG. 2, there is illustrated said mold 1, the cavity 4 of which'has been filled with the aforementioned powder mixture, here indicated at 6, of emissive material and metal, and in this example, barium peroxide and tantalum. A metal lead 7 which is to serve as an electrode conducting lead and which may be made of a suitable conducting material, such as nickel, tungsten, tantalum iron and alloys thereof, has been positioned within hole 5. The diameter of lead 7 is approximately twenty-thousandths of.an inch and its length is three-eighths of an inch, the lead therefore extending one-eighth of an inch into cavity 4 where it is surrounded by powder mixture 6. The uppermost portion of lead 7is formed with a nailhead 8 having a diameter of approximately one thirty-second of an inch, this nailhead being provided to improve the bonding between lead 7 and the fused pellet electrode which is to be formed.

After powder mixture 6 is placed in cavity 4 it is compressed, with a pressure of LOGO-4,000 pounds per square inch, by, for example, a weighted steel plunger.

The quantity of powder placed in said cavity, between 150-250 milligrams, is that amount needed to fill said cavityto a level approximately one thirty-second of an inch below the top thereof after said mixture has been compressed. Itis appropriate here to note that the pressure with which mixture 6 is compressed, while not critical, as evidenced by the fact that the suitable compression pressure may range between LOGO-4,000 pounds per square inch, is important. As discussed above, the rate of exothermic reaction controls the density gradient of the completed electrode and it will-be clear to those skilled in the art that the degree of compression to which the powder mixture is subjected will affect the rate of the aforementioned exothermic reaction.

A cover 9, which is made of the same material as the mold, here graphite, and having a somewhat concave lower surface, is placed over cavity 4 containing powder mixture 6. Cover 9 formed with a vent hole 10' in the central portion thereof, said vent hold being approximately one thirty-second of an inch in diameter, and it is provided for the purpose of allowing gases, expunged from powder mixture 6 during the exothermic reaction which is to be discussed below, to escape.

Mixture 6 may now be heated to initiate the desired exothermic reaction between the tantalum and barium peroxide. The heat necessary to start said exortherrnic reaction may be provided in'a number of ways, for example, by a muffle furnace. It may also be provided by the structure, here illustrated, which includes an RF work coil 11 surrounding mold 1, said coil 11 being connected to a source of electrical energy 12 by conductors 13. To begin the exothermic reaction it is necessary to heat mixture 6 to a temperature between 700 and 1,000C, said temperature being above the melting temperature of the barium peroxide powder and the temperature at which the exothermic reaction will begin within the mold. To provide the required heating,

taking into account the impedance of the mold arid-the material which is to be heated, the source of electrical energy 12, here illustrated, is selected to "operate at a frequency of 450 kilohertz and to pro'vide'a current of approximately 165 milliamperes. Source 12 will remain energized until the exothermic reaction begins, said reaction being observable through hole 10 as a flash of light. Once the exothermic reaction begins source 12 can be de-energized since the exothermic reaction will continue until it self extinguishes, the duration of the reaction being determined by the quantity of the mixture present within mold 1; After the exothermic reaction hasended and the completed fused electrode has been cooled, it is available for standard processing and subsequent use in a fluorescent lamp.

Referring now to FIG. 3, there is illustrated the completed fused pellettype cold cathode having a bulk density gradient structure. Cathode 14 has a somewhat conical top surface 115 and cylindrical sides 16. Conical surface 15 has a plurality of jagged protrusions extending therefrom and a void 17 exists in the interior of cathode 14 at the upper surface of lead 7 due to the outward explosion of the pellet material caused by the above-mentioned exothermic reaction. Cathode 14 has, as previously stated, a bulk density gradient structure, by which is meant that the top portion 15 comprises particles 25 to 50 microns in size and 80 percent voids, while the exterior sides and bottom and deeper interior portion of fused pellet electrode 14 comprises particle sizes in the order of tenths of microns and approximately 10 percent voids.

The particular structure and configuration of cath-- ode M is due to the size and shape of the compressed powder mixture 6, the fact that it was closely confined within cavity 4, the vent path provided for the gases expunged from compressed mixture 6 due to the abovediscussed exothermic reaction, the concave contour of the underside of cover 9 and, of course, the outwardly directed force provided by said exothermic reaction.

Cathode 34, as discussed above, has both a density gradient structure, which makes it easier for electrons to travel through the cathode material to the surface thereof, and a plurality of jagged protrusions extending from said surface which serve to reduce the time duration of the glow to arc transition to less than one-half second. These two factors aid in lowering the ignition voltage of the lamp in which the cathode is utilized, said ignition voltage having been experimentally determined to be approximately 400 to 425 volts as compared to the approximately 400 to 450 volts required by the fused pellet electrode disclosed in the aforementioned related application Ser. No. 204,469. Further, the reduction of the time duration of the glow to arc transition, due to the aforementioned jagged protrusions, serves to reduce the quantity of the cathode material sputtered, as does the fact that the bottom and side surfaces of cathode R4 are smooth and dense relative to that of top surface 15, since electron emission will now take place only at surface 15. The sputtering of electrode M is thus substantially less than that of the electrode described in the aforementioned related application and this results in less of the previously discussed undesirable end-blackening. Electrode 14 has also been determined to be structurally sounder than the pellet described in said related application due to the fact that the bottom and side surfaces thereof are smooth and dense rather than jagged and porous as was the case with said related cathode. Further, the use of lead 7 having a nailhead 8 at the end thereof has resulted in an electrical connection between the lead and the pellet superior to the connection obtained with the structure described in said related application. Additionally it will be noted that a relatively large amount of emissive material, here alkaline earth material, has been used in manufacturing cathode l4 and it will therefore have an extremely long life span relative to that of hot or hybrid cathodes.

It will thus be seen that there has been provided an emissive fused pellet electrode suitable for use in fluorescent lamps which is superior to those cathodes presently known in the art.

it is here appropriate to note that the subject cathode structure has been discovered to be relatively air stable subsequent to its activation, that is, it will remain activated for a period of at least one hour when maintained after said activation in reasonably dry air, by which is meant air containing less than grains of water per pound of dry air. Thus, the cathode here described,

while being greatly improved over the cathodes known in the art, for the reasons previously discussed, additionally is suitable for batch processing, thus providing an additional valuable advantage.

While the principles of the invention have been described in connection with specific structures, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention as set forth in the objects thereof and in the accompanying claims.

What is claimed is:

l. A fused pellet electrode comprising:

a fused pellet having a top, a bottom and sides and containing a fused mixture of electron emissive material and a metal; and

a metal conducting lead embedded in and extending from the bottom of said pellet, said pellet having a bulk density gradient structure,

wherein the interior portions and exterior bottom and side portions of said pellet have a high bulk density relative to the bulk density of the exterior top portion of said pellet.

2. A fused pellet electrode, according to claim 1, wherein the bottom and side surfaces of said pellet are smooth relative to the top surface of said pellet so as to confine electron emission to the top portion of said pellet.

3. A fused pellet electrode, according to claim 2, wherein the top portion of said pellet is conical in shape and the bottom portion of said pellet is cylindrical in shape, the base of said conical portion being adjacent said cylindrical portion.

4. A fused pellet electrode, according to claim 3, wherein said metal comprises a refactory metal.

5. A fused pellet electrode, according to claim 3, wherein said metal comprises a transition element metal.

6. A fused pellet electrode, according to claim 3, wherein said electron emissive material comprises an alkaline earth material.

7. A fused pellet electrode, according to claim 3, wherein said electron emissive material comprises an alkali material.

8. A fused pellet electrode, according to claim 3, wherein that portion of said metal conducting lead embedded in said pellet is formed with anail head so as to provide good adherence between said lead and said fused pellet.

9. A fused pellet electrode, according to claim 3, wherein said fused pellet results from an exothermic reaction in a powder mixture comprisinga powder of said metal and a powder of said electron emissive material.

10. A fused pellet electrode, according to claim 9, wherein said metal powder comprises tantalum powder and said electron emissive material comprises barium peroxide powder, l

said tantalum powder being between 60 percent and percent by weight of said powder mixture and said barium peroxide powder being between 30 percent and 40 percent by weight of said powder mixture.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,758,809 Dated September 11, 1973 Inventor) Richard A. Menelly Edmund R, Kern I It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, first paragraph, line 2, delete "Patent No. 3,502,855"

andreplace with the words --now abandonedsigned and sealed this" 16th day or July 1974.

(SEAL) Attest:

MCCOY M. GIBSON, JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents

Claims

2. A fused pellet electrode, according to claim 1, wherein the bottom and side surfaces of said pellet are smooth relative to the top surface of said pellet so as to confine electron emission to the top portion of said pellet.
3. A fused pellet electrode, according to claim 2, wherein the top portion of said pellet is conical in shape and the bottom portion of said pellet is cylindrical in shape, the base of said conical portion being adjacent said cylindrical portion.
4. A fused pellet electrode, according to claim 3, wherein said metal comprises a refactory metal.
5. A fused pellet electrode, according to claim 3, wherein said metal comprises a transition element metal.
6. A fused pellet electrode, according to claim 3, wherein said electron emissive material comprises an alkaline earth material.
7. A fused pellet electrode, according to claim 3, wherein said electron emissive material comprises an alkali material.
8. A fused pellet electrode, according to claim 3, wherein that portion of said metal conducting lead embedded in said pellet is formed with a nail head so as to provide good adherence between said lead and said fused pEllet.
9. A fused pellet electrode, according to claim 3, wherein said fused pellet results from an exothermic reaction in a powder mixture comprising a powder of said metal and a powder of said electron emissive material.
10. A fused pellet electrode, according to claim 9, wherein said metal powder comprises tantalum powder and said electron emissive material comprises barium peroxide powder, said tantalum powder being between 60 percent and 70 percent by weight of said powder mixture and said barium peroxide powder being between 30 percent and 40 percent by weight of said powder mixture.